MIT engineers have built a new desalination system that runs with the rhythms of the sun. Because it doesn’t need expensive energy storage for times without sunshine, the technology could provide communities with low-cost drinking water.
The solar-powered system removes salt from water at a pace that closely follows changes in solar energy. As sunlight increases through the day, the system ramps up its desalting process and automatically adjusts to any sudden variation in sunlight, for example, by dialling down in response to a passing cloud or revving up as the skies clear.
Because the system can quickly react to subtle changes in sunlight, it maximises the utility of solar energy, producing large quantities of clean water despite variations in sunlight throughout the day. In contrast to other solar-driven desalination designs, the MIT system requires no extra batteries for energy storage nor a supplemental power supply, such as from the grid.
The engineers tested a community-scale prototype on groundwater wells in New Mexico over six months, working in variable weather conditions and water types. The system harnessed, on average, over 94 per cent of the electrical energy generated from the system’s solar panels to produce up to 5,000l of water per day despite large swings in weather and available sunlight.
“Conventional desalination technologies require steady power and battery storage to smooth out a variable power source like solar. By continually varying power consumption in sync with the sun, our technology directly and efficiently uses solar power to make water,” Germeshausen Professor of Mechanical Engineering and Director of the K. Lisa Yang Global Engineering and Research (GEAR) Center at MIT, Amos Winter said.
“Being able to make drinking water with renewables, without requiring battery storage, is a massive grand challenge. And we’ve done it.”
The system is geared toward desalinating brackish groundwater — a salty water source found in underground reservoirs and is more prevalent than fresh groundwater resources. The researchers see brackish groundwater as a vast untapped source of potential drinking water, particularly as freshwater reserves are stressed worldwide. They envision that the new renewable, battery-free system could provide much-needed drinking water at low costs, especially for inland communities with limited access to seawater and grid power.
“Most of the population lives far enough from the coast that seawater desalination could never reach them. They consequently rely heavily on groundwater, especially in remote, low-income regions. And unfortunately, this groundwater is becoming more and more saline due to climate change,” MIT PhD student in mechanical engineering Jonathan Bessette said.
“This technology could bring sustainable, affordable clean water to underreached places worldwide.”
Pump and flow
Electrodialysis and reverse osmosis are the two main methods for desalinating brackish groundwater. With reverse osmosis, pressure is used to pump salty water through a membrane and filter out salts. Electrodialysis uses an electric field to draw out salt ions as water is pumped through a stack of ion-exchange membranes.
Scientists have looked to power both methods with renewable sources. However, this has been incredibly challenging for reverse osmosis systems, which traditionally run at a steady power level incompatible with naturally variable energy sources such as the sun.
The team focused on electrodialysis, seeking ways to make a more flexible, “time-variant” system responsive to variations in renewable solar power.
In their previous design, the team built an electrodialysis system consisting of water pumps, an ion exchange membrane stack, and a solar panel array. The innovation in this system was a model-based control system that used sensor readings from every part of the system to predict the optimal rate at which to pump water through the stack and the voltage that should be applied to the stack to maximise the amount of salt drawn out of the water.
When the team tested this system in the field, its water production could vary with the sun’s natural variations. On average, the system directly used 77 per cent of the available electrical energy produced by the solar panels, which the team estimated was 91 per cent more than traditionally designed solar-powered electrodialysis systems.
Image: Haider Y. Abdulla/stock.adobe.com
